plant responses

What is abiotic stress?

Non-living environmental factor that could harm a plant e.g light, water and mineral nutrient shortage

How do plant responses help them survive?

It helps them avoid predation (grazing), avoid abiotic stress and find their preferred conditions

Outline the three key responses to abiotic stress and herbivory in plants:

Chemical defences, folding in responses to touch and tropisms

What are the 3 key categories of chemical defences in plants

Tannins, alkaloids and pheromones

How do tannins work?

They taste bitter and in some herbivores can bind to proteins and inactivate digestive enzymes making it hard to digest

How do alkaloids work?

They are chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores

How do pheromones work?

They may be released to signal the presence of herbivores to neighbouring plants or species. They can they fortify their defences e.g release callose for protection

What plant folds in response to touch?

Mimosa pudica

How does folding in response to touch help plants survive?

It may help to knock off small insects and may scare off animals trying to eat it

What are nastic movements and how are they different to tropisms?

They are non-directional responses to stimuli and involve changes in the turgidity of specialised cells

What is a tropism?

A growth response where the direction of growth is determined by the direction of the stimulus

What is faster: a nastic response or a tropism?

A nastic response is faster

What are the four types of tropism we need to know?

Phototropism, geotropism, chemotropism and thigmotropism

What is thigmotropism?

A growth response to touch

What type of geotropism do roots display and how is auxin involved?

They are positively geotropic - auxin inhibits growth of root cells on the underside

What type of geotropism do shoots display and how is auxin involved?

They are negatively geotropic - auxin stimulates growth of shoot cells on the underside

How does positive thigmotropism work?

The cells that touch the structure produce auxin, which stimulates cells to grow/elongate more. Hence the tendrils coil around the structure that is touched

Give two differences between animal hormones and plant hormones

Plant hormones are not made in glands and they are not transported over distance in vascular tissue

How do plant hormones work?

They bind to receptors in their target cells to bring about their affect

How can plant hormones have very different effects in different tissues, and in the same tissue?

The effects depend on the age of the plant and the presence of other hormones

What are the 5 key plant hormones we need to know?

Cytokinins, abscisic acid, auxins, gibberellins and ethene

What three roles does auxin have?

It is involved in phototropism, geotropism and apical dominance

How does auxin affect phototropism?

It moves to the more shaded part of the shoots and roots, causing the cell to elongate and bend towards the light

How does auxin affect geotropism in shoots?

It moves to the underside of shoots, causing the cell to elongate and the shoot grows upwards

How does auxin affect geotropism in roots?

It moves to the underside of roots, inhibiting growth and so the shoot grows downwards

How is auxin transported?

Diffusion, active transport and via the phloem over long distances

What is apical dominance?

The apex of an actively growing plant controls the outgrowth of lateral buds and is more dominant.

How are auxins involved in apical dominance?

They stimulate the growth of the apical bud and inhibit the growth of side shoots from lateral buds

Why is apical dominance useful in plants?

Energy isn't being used to grow side shoots so it allows a plant to grow tall very fast and reach the sunlight.

What happens if the apex is removed or not active?

Auxin concentration is lower so inhibition of growth is removed - side buds grow

Why might very tall plants have side shoots at the bottom?

Auxins become less concentrated as they move away from the apical bud, so the bottom of a very tall plant will have a low auxin conc and so side shoots might grow

What is the role of cytokinins?

They stimulate cell division and growth

What is the role of gibberellins?

They are involved in seed germination and stem elongation

How do gibberellins work?

They increase the synthesis of enzymes to enable the response to occur

What happens if a dwarf variety of a plant is treated with gibberellins?

It will grow to the same height as the tall variety.

Can gibberellins inhibit growth?

No.

What is the role of abscisic acid?

It is involved in stomatal closing in response to water stress

What is the role of ethene?

It is a gas which promotes fruit ripening

What is a coleoptile

a sheath that encloses the shoot of the plant embryo

How can you investigate apical dominance?

You can plant 30 plants which are a similar age, height and weight. Count and record the number of side shoots in each plant. For 10 of the plants, remove the tip of the shoot and apply a paste containing auxins to the stem. For 10 of the plants, remove the tip of the shoot and apply a paste without auxins to the stem. Leave the final 10 plants as a control. Leave the plants for six days and count the number of side shoots growing.

How do we know that auxin is diffusible and water soluble?

Investigations using mica inserted into a coleoptile show there is no response to light. When the tip was cut and replaced with a gelatine block, the coleoptile curves towards light

Give 3 commercial uses of auxin

Rooting powder, producing seedless fruit and selective herbicide

How are auxins used in rooting powders?

Rooting powders contain auxin which stimulates root initiation, and a fungicide to prevent fungal disease

How do auxins produce seedless fruit?

There is no fertilisation - spraying with auxin stimulates ovule development which allows fruit formation without seeds as no fertilisation has occurred

How can auxins work as selective herbicides?

They can cause excessive growth in dicot plants, causing them to die

How do gibberellins allow stems to elongate?

The cell walls become more plastic and can stretch when water is absorbed by osmosis

How can fruits be commercially ripened?

Fruits can be picked unripe and then sprayed with ethene at their destination

What effects (3) does the release of adrenaline have in the fight-or-flight response?

Causes an increase in HR and stroke volume, stimulates the breakdown of glycogen into glucose in the liver cells and increases the diameter of the bronchioles by causing the smooth muscles in the airways to relax

What effects (3) does the release of cortisol have in the fight-or-flight response

Increase blood pressure, suppresses the immune system and slows functions neural functions so that the brain can focus on the threat